Cheap alternatives to translucent polycarbonate sheet

As you have already found out, cutting P-glass is tricky. The plastic cutters you mentioned are very gimmicky and don't work that well. I would suggest the following procedure:

After marking out your cut on the plexi, take a scrap piece of any firm material, plywood, an old board, anything, and cut a slightly oversuninstallized hole in it to act as a backer for your plexi.

With your plexi on top of the backer, Drill holes in two oposite corners of the marked hole you want to make.

Use a varible speed jig saw at a very low speed with a fine tooth wood or course tooth metal blade to cut out the shape. Stay just barely inside of the waste side of the mark.

Finish the hole and smooth the edges with a fine bastard file and some 220 grit paper.

If you are really cleaver, you could make the hole smaller than the device cover so no plexi edges would be exposed. You may need longer screws to attach the cover plates, but you can get them at any electrical dept.

You do not specify thickness, so I'll take a guess at 1/4" or thinner.

The problem with drilling plastic is that conventional bits will "catch" on the plastic, pulling the bit into the hole with more force than desired, typically cracking the plastic being drilled.

Using step-up method to drill the hole will not eliminate this problem.

Lacking the aforementioned hole saw, the spade bit is a better choice. I have Forstner bits but have not considered trying one on plexiglass. If you have a drill press and the increased control it offers, you may have good luck taking small bites with the forstner. The cutting edge of a forstner bit is more like the edge of a plane and may also dig in too quickly otherwise.

Drill bits designed for plastic have a much sharper angle. A set I recently purchased has an abrasive material on the cutting surface as well as a subdued cutting angle, effectively abrading the plastic and lifting it to the surface.

I overlooked your option of a step drill which is the best of the list. Each progressive cutting edge will not gouge into the plastic as much as any of the other available choices. Again, a drill press improves your chances of a good result.